Field of the Invention
The present invention relates to an image forming apparatus, which is configured to correct distortion and uneven image density of an image during image formation of a two-dimensional image by the image forming apparatus, e.g., a digital copying machine, a multifunctional peripheral, or a laser printer.
Description of the Related Art
In electrophotographic image forming apparatus such as a laser printer and a copying machine, there has been generally known a configuration to form a latent image on a photosensitive member with the use of a light scanning device configured to perform scanning with a laser beam. In the light scanning device of a laser scanning type, a laser beam collimated with the use of a collimator lens is deflected by a rotary polygon mirror, and the deflected laser beam is formed into an image on a photosensitive member with the use of an elongated fθ lens. Further, there is known multibeam scanning in which a laser light source having a plurality of light emission points is included in one package so as to perform scanning with a plurality of laser beams simultaneously.
Meanwhile, in order to form a satisfactory image without uneven image density and banding, it is desired that distances between scanning lines of which positions to be scanned with a laser beam are adjacent to each other in a rotational direction of the photosensitive member be equal to each other. However, the distances between the scanning lines are varied due to a plurality of factors described below. The distances between the scanning lines on the photosensitive member are varied by, for example, a fluctuation in a surface speed of the photosensitive member, or a rotation speed fluctuation of a rotary polygon mirror. Further, the distances between the scanning lines are also varied by a variation in angle of mirror faces of the rotary polygon mirror with respect to a rotary shaft of the rotary polygon mirror and a variation in intervals between light emission points arranged on a laser light source. FIG. 19A is an illustration of a state in which an interval between the scanning lines is varied periodically, with scanning of laser beams being represented by horizontal lines. As illustrated in FIG. 19A, when the interval between the scanning lines of laser beams is small, an image is developed darkly. When the interval between the scanning lines of laser beams is large, an image is developed lightly. Thus, the development is liable to be detected as moire and the like. To cope with uneven image density and banding caused by such factors, there has been proposed a technology of correcting banding by controlling an exposure amount of the light scanning device. For example, in Japanese Patent Application Laid-Open No. 2012-098622, there is described a configuration in which a beam position detection unit configured to detect a beam position in a sub-scanning direction is arranged in the vicinity of the photosensitive member, and the exposure amount of the light scanning device is adjusted based on scanning distance information obtained from a detected beam position, to thereby make banding less noticeable.
Further, the image forming apparatus subjects image data to halftone processing with the use of a dither pattern, to thereby express a halftone (intermediate tone). In an image subjected to the halftone processing, for example, a line screen or a dot screen is used.
However, as a screen to be used in the halftone processing, there is given a screen that is liable to be influenced by a mirror face tangle error of a rotary polygon mirror (hereinafter simply referred to as “optical face tangle error of a rotary polygon mirror”) and a screen that is less liable to be influenced by the optical face tangle error of a rotary polygon mirror. FIG. 19B and FIG. 19C are each a view for illustrating a phenomenon of the optical face tangle error of a rotary polygon mirror. In FIG. 19B and FIG. 19C, a gray portion represents a dither pattern. Further, a light gray portion (white portion) represents an area in which the interval between scanning lines of a laser beam emitted from a light source is sparse, and a dark gray portion (black portion) represents an area in which the interval between scanning lines is dense. In an image using the line screen of FIG. 19B, streaks of the line screen regularly extend in an area of the occurrence of sparseness and denseness of the scanning lines, and hence moire appears strongly. Meanwhile, in an image using the dot screen of FIG. 19C, as compared to the line screen, dots irregularly overlap an area of the occurrence of sparseness and denseness, and the occurrence frequency of shading difference is lower than that of the line screen, with the result that the intensity of moire is smaller than that of the line screen.
Further, when an exposure amount is controlled for correcting uneven image density caused by sparseness and denseness of the scanning lines, the density per predetermined area is not stored before and after correction, and hence correction does not act well depending on a pattern of an input image, and the performance of correction may be degraded. In FIG. 20A to FIG. 20C, the horizontal direction represents a main scanning direction, and the vertical direction represents a sub-scanning direction. Further, each rectangle represents a scanning trajectory of a laser beam, and the darkness of the rectangle represents the intensity of lighting of a laser beam. In this case, first to third faces of the mirror faces of a rotary polygon mirror are expressed in a unit of four beams downwardly. As compared to FIG. 20A in which no optical face tangle error of the rotary polygon mirror occurs, an optical face tangle error that is shifted upward occurs in FIG. 20B. When the conventional correction is performed with respect to FIG. 20B, results as illustrated in FIG. 20C are obtained. In FIG. 20C, density is decreased in areas A0 and A1 as a result of the correction, and density is increased in areas B0 and B1 as a result of the correction. Therefore, correction cannot be performed appropriately.
In view of the above-mentioned conventional method, there is considered a method involving correction of sparseness and denseness by moving a center of gravity of density across a plurality of pixels. However, in the method involving movement of a center of gravity of density, there is also a risk in that the effects of correction may not be obtained when the moved density cannot be reproduced with high accuracy in accordance with the tone characteristics. Further, a change with time, and environmental fluctuations, e.g., fluctuations in temperature and humidity, greatly influence electrophotographic tone characteristics. For example, even when appropriate correction as shown in FIG. 21B is performed, the performance of the correction may be degraded as shown in FIG. 21C and FIG. 21D depending on environmental change and the like. Therefore, there is a demand for performing correction appropriately even when tone characteristics are changed due to environmental fluctuations.